The present invention relates generally to industrial sewing machines, and in particular to a speed setting device for such sewing machines.
Industrial sewing machines includes a motor and a control box fixed to the underside of the motor. An operator-controlled lever is pivotally mounted on one side wall of the control box, one end of the pivoted lever being connected by a pitman i.e., rod connecting to a foot pedal of the sewing machine. The angular displacement of the foot pedal is transmitted via the lever to inside of the control box where a swingable arm is located. The free end of the arm is shaped as an arch adjacent to which is located an angular displacement sensor for generating a speed setting signal in response to the rotation of the arm. The speed setting signal is used to control the speed of the motor to effect various automatic functions including variable speed control, needle position detection and stoppage and thread cutting operation. One disadvantage of the conventional speed setting device is that the lever and the arm must be rigidly coupled together with such a strength that there is no slippage at the point of coupling. If this strength is not sufficient, the slippage will result in an error in the speed setting signal. Another disadvantage relates to the fact that the maximum range of angular displacement is not sufficient for the displacement sensor to generate a position signal which is an accurate representation of the angular position of the foot pedal. As a result, an extra device is needed for amplifying the amount of angular displacement of the arm.